The present invention relates to an optical device of which optical elements are bonded without using adhesive, and a method for producing an optical device.
In recent years, an optical communication system has become of high integration according to increasing the number of wavelength in WDM (Wavelength Division Multiplex). Consequently, demand on miniaturization of an optical device used for it has also been increased. In many cases, optical devices are composed of combinations of bonded bodies formed by bonding optical elements such as a Faraday rotator and polarizers to a fixing member. However, according to this method, a fixing member is an obstacle, which hinders the miniaturization of optical devices. Accordingly, it has been considered a method such that a fixing member is omitted and optical elements are bonded to each other.
The easiest method for bonding optical elements each other is to bond them by using organic adhesive. For example, Japanese Patent Laid-open Publication No. 6-75189 discloses an optical isolator wherein optical elements are adhered each other using organic adhesive such as resin to be unified. However, it has a disadvantage that use of the organic adhesive causes generation of outgas, which adversely affects a laser diode. Furthermore, the organic adhesive is easily affected by irradiation of high energy laser and exposure to atmosphere of high temperature and high humidity, and therefore it may causes low reliability to the device.
Accordingly, there have been investigated various methods of bonding optical elements each other without using organic adhesive. For example, there is a method of bonding optical elements by using low-melting glass or solder as inorganic bonding material. Low-melting glass is a glass for bonding of which the main component is a low-melting point material such as B2O3, PbO, or the like. It is necessary to heat to higher temperature than the softening point of the glass at a time of bonding using it. Moreover, although it is effective to bond light transmissive surfaces of the optical elements each other in order to achieve miniaturization of an optical device, there may be caused a problem when bonding the light transmissive surfaces of the optical elements using such low-melting glass, an antireflection film formed on the optical elements may react with the low-melting glass during softening of the low-melting glass by heating, which may lead to lowering of antireflection function. For this reason, it has been considered that a practical application of the optical device obtained by using low-melting glass for bonding each light transmissive surface was difficult.
On the other hand, in the case of using solder, since solder has no transparency, it can not be disposed directly on each light transmissive surface Therefore, such a bonding method that each outer frame of light transmissive surfaces is selectively metalized to exist solder only on the metalized surface is employed. Such a bonding method suffers from a problem that a complicate metalizing process is required, and therefore, decrease in yield and increase in cost can not be avoided.
Moreover, a method that each optical element is directly bonded without using adhesive has been attempted. (See Japanese Patent Application Laid-open (kokai) No. 7-220923 and Japanese Patent Application Laid-Open Application (kokai) No. 2000-56265.) In these methods, after surfaces of optical elements are subjected to hydrophilic treatment, hydrophilic-treated surfaces are bonded each other. This method is practically used for a manufacturing process of an SOI (Silicon On Insulator) wafer in the semiconductor field. However, in the case of applying this method to an optical device, it suffers from problems as follows and therefore it is a difficult situation to put this method to practical use.
Namely, such a method wherein the optical elements are subjected to hydrophilic treatment and then bonded directly depends on a configuration and physical properties of the components to be bonded. For example, as for the warp, the curvature radius is desirably several hundreds meters or more. Moreover, it is said that surface roughness of components to be bonded is desirably Ra=0.3 nm or less. Furthermore, it is greatly influenced by difference in linear expansion coefficient of components to be bonded.
However, only few optical elements satisfy the above-mentioned requirements. For example, since an iron garnet crystal or the like which is one of optical elements generally used in the optical device has stress distribution in thickness direction, it often has large warp. Moreover, since a polarizing glass has the structure wherein metal particles such as silver, copper or the like are dispersed in glass, surface roughness thereof is hardly controlled. Furthermore, linear expansion coefficients of these optical elements often differ greatly depending on material, and thus there is a tendency that the difference in the linear expansion coefficient between components to be bonded becomes large. Therefore, the optical elements bonded directly as mentioned above are easily delaminated at the bonded surface when they are subjected to heat treatment, and adhesiveness and durability of the bonded surface are low.
Furthermore, there is a problem that when the materials of which linear expansion coefficient are different from each other are bonded directly as above, thermal stress is generated between different materials, and it is concentrated on the bonded surface, and thereby optical strain may easily generate, resulting in lowering of optical properties such as the extinction ratio. Therefore, it is very difficult to apply a direct bonding technique to an optical device.
As described above, it has been very difficult to bond optical elements without using organic adhesive, and to produce an optical device having high reliability easily at a low cost.
The present invention has been accomplished to solve the above-mentioned previous problems. An object of the present invention is to provide an optical device which has small size and has high reliability at a low cost by bonding optical elements each other without using organic adhesive.
To achieve the above mentioned object, the present invention provides a method for producing an optical device by bonding optical elements each other without using adhesive wherein the optical elements are bonded each other
by using optical elements in which the relation between the linear expansion coefficient xcex11 and xcex12 (/xc2x0 C.) of each of the optical elements to be bonded and the thickness t2 (m) of one of the optical elements satisfies the following formula; |(xcex11xe2x88x92xcex12)xc3x97t2|xe2x89xa610xe2x88x929 and t2xe2x89xa72xc3x9710xe2x88x925;
and/or, by sticking the optical elements each other in the state of being heated, and then subjecting them to a heat treatment.
According to the method of producing an optical device having such features, optical elements can be bonded with sufficient bonding strength without using adhesive. Moreover, since organic adhesive is not used, there is caused neither generation of outgas nor degradation of the bonded surface due to atmosphere. Therefore, a small size optical device having excellent optical properties and high reliability can be produced at a low cost.
Moreover, in order to accomplish the above object, according to the first embodiment of the present invention, there is provided an optical device formed by bonding a polarizer to at least one surface of a magnetic garnet crystal without using adhesive which functions by transmitting light through the bonded surface, wherein the relation of the linear expansion coefficient xcex11 (/xc2x0 C.) of the magnetic garnet crystal, the linear expansion coefficient xcex12 (/xc2x0 C.) of the polarizer and the thickness t2 (m) of the polarizer satisfies the following formula: |(xcex11xe2x88x92xcex12)xc3x97t2|xe2x89xa610xe2x88x929 and t2 xe2x89xa72xc3x9710xe2x88x925.
As described above, if the relation between the linear expansion coefficient xcex11 (/xc2x0 C.) of the magnetic garnet crystal, the linear expansion coefficient xcex12 (/xc2x0 C.) of the polarizer, and the thickness t2 (m) of the polarizer satisfies the following formula: |(xcex11xe2x88x92xcex12)xc3x97t2|xe2x89xa610xe2x88x929 and t2xe2x89xa72xc3x9710xe2x88x925, delamination of the magnetic garnet crystal and the polarizer in heat treatment process for bonding can be prevented, and sufficient bonding strength can be achieved. Moreover, since the thermal stress generated between the magnetic garnet crystal and the polarizer can be reduced, degradation of the optical properties due to the optical strain originated from thermal stress can be suppressed. Furthermore, since organic adhesive is not used, there is neither generating of outgas nor degradation of a bonded surface due to it. Therefore, a small size optical device which has excellent optical properties and high reliability can be provided at a low cost.
In this case, a metal oxide film is preferably formed on the surface to be bonded to the polarizer of the above-mentioned magnetic garnet crystal. It is preferable that the metal oxide film consists of one kind or two or more kinds of metal oxide films selected from Al2O3, TiO2 and SiO2, and the metal oxide film has a structure laminated in a single-layer or a multilayer.
As described above, if the metal oxide film is formed on the surface to be bonded to the polarizer of the magnetic garnet crystal, it can act as an antireflection film and bonding to a polarizer can be stronger. Moreover, if the metal oxide film consists of one kind or two or more kinds of metal oxide films selected from Al2O3, TiO2 and SiO2, and it has a structure laminated in a single-layer or a multilayer, it is excellent as an antireflection film, and can achieve significant improvement in bonding strength. Thus, the optical device having high performance and high reliability can be obtained.
Moreover, the above-mentioned polarizer is a preferably a polarizing glass in that case.
One of the requirements of the present invention is to set up a thickness of a polarizer appropriately as described above. Therefore, it is required that the optical properties of a polarizer hardly depend on a thickness thereof. Therefore, it is preferable that a polarizer is a polarizing glass wherein optical properties are influenced little by a thickness. Thereby, the thickness of the polarizer can be set up appropriately, without degrading optical properties.
Furthermore, it is preferable that the above-mentioned magnetic garnet crystal is a bismuth-substituted iron garnet crystal.
As described above, if the magnetic garnet crystal is a bismuth-substituted iron garnet crystal which is excellent in Faraday-rotation ability, the Faraday-rotation angle of 45 degrees can be realized with a thickness of about 0.5 mm, and thus it is effective for miniaturization of an optical device.
Moreover, the above mentioned optical device can be an optical isolator.
The optical isolator is one of the most useful optical devices, and it is an indispensable device in optical communication. Thus, when the optical device of the present invention is an optical isolator, there can be provided an optical device which can meet miniaturization of an optical isolator and an organic adhesive free optical device strongly requested in recent years.
Then, a method for producing an optical device according to the first embodiment of the present invention is a method for producing an optical device by bonding a polarizer to at least one surface of a magnetic garnet crystal without using adhesive wherein the bonding is performed by using the magnetic garnet crystal and the polarizer in which the relation between the linear expansion coefficient xcex11 (/xc2x0 C.) of the magnetic garnet crystal, the linear expansion coefficient xcex12 (/xc2x0 C.) of the polarizer, and the thickness t2 (m) of a polarizer satisfies the following formula:
|(xcex11xe2x88x92xcex12)xc3x97t2|xe2x89xa610xe2x88x929 and t2xe2x89xa72xc3x9710xe2x88x925. 
As described above, if an optical device is produced by using the magnetic garnet crystal and the polarizer in which the relation between the linear expansion coefficient xcex11 (/xc2x0 C.) of the magnetic garnet crystal, the linear expansion coefficient xcex12 (/xc2x0 C.) of the polarizer, and the thickness t2 (m) of the polarizer satisfies the following formula:
|(xcex11xe2x88x92xcex12)xc3x97t2|xe2x89xa610xe2x88x929 and t2xe2x89xa72xc3x9710xe2x88x925, the magnetic garnet crystal and the polarizer can be bonded with sufficient bonding strength without using adhesive, and degradation of the optical properties by the optical strain in a bonded body can be suppressed. Therefore, the optical device with high reliability having excellent optical properties can be manufactured at a low cost.
In this case, the above-mentioned magnetic garnet crystal and the above-mentioned polarizer are bonded preferably by subjecting each of the bonded surfaces to polishing, cleaning, hydrophilic treatment and drying processes, and then sticking the bonded surfaces directly or through solution, followed by subjecting them to heat treatment.
As described above, if each of the bonded surfaces of the magnetic garnet crystal and the polarizer is subjected to polishing, cleaning, hydrophilic treatment and drying processes, and then they are stuck directly or through solution, and subjected to heat treatment, chemical species which constitute the magnetic garnet crystal and chemical species which constitute the polarizer can interact effectively, and sufficient bonding strength can be achieved. Thereby, delamination of a bonded surface can be prevented.
In this case, it is still more preferable that as the above-mentioned solution used when the magnetic garnet crystal and the polarizer are bonded, solution containing as a main component polar molecules is used independently or by mixture.
As described above, if the solution containing as a main component polar molecules is used independently or by mixture when the magnetic garnet crystal and the polarizer are bonded, bonding strength between the magnetic garnet crystal and the polarizer can be further improved.
Moreover, it is preferable that the magnetic garnet crystal and the polarizer are bonded after forming a metal oxide film on the bonded surface of the magnetic garnet crystal which is to be bonded to the polarizer.
As described above, by bonding them after forming a metal oxide film on the surface of the magnetic garnet crystal which is to be bonded to the polarizer, the bonding strength can be further improved. Moreover, since the formed metal oxide film functions as an antireflection film in an optical device, an optical device which is highly reliable and highly efficient can be manufactured.
Furthermore, it is preferable that the metal oxide film formed on the magnetic garnet crystal consists of one kind or two or more kinds of metal oxide films selected from Al2O3, TiO2 and SiO2, and the metal oxide film is laminated in a single-layer or a multilayer.
As described above, if the metal oxide film consists of one kind or two or more kinds of metal oxide films selected from Al2O3, TiO2 and SiO2, and it is laminated in a single-layer or a multilayer, the function of a metal oxide film as an antireflection film can be improved further, and the bonding strength of the magnetic garnet crystal and the polarizer can also be increased remarkably.
Moreover, according to the present invention, an optical isolator can be manufactured by bonding the polarizer to the magnetic garnet crystal.
If an optical isolator is manufactured as described above, a small size optical isolator which has sufficient bonding strength can be manufactured without using adhesive.
Furthermore, in order to achieve the above-mentioned purpose, according to the second embodiment of the present invention, there is provided a method for producing an optical device by bonding optical elements each other without using adhesive in which the optical elements are bonded by at least subjecting the bonded surface of each of the optical elements to polishing, cleaning, and hydrophilic treatment, and then sticking the bonded surfaces of the optical elements each other, followed by subjecting them to heat treatment, wherein the optical elements are stuck each other in a state of being heated, and then subjected to the heat treatment.
As described above, if the optical elements are heated and then the heated optical element are stuck each other, the difference between the temperature of the optical elements when being stuck and the temperature thereof at the heat treatment can be made small. And thereby, thermal stress generated on the bonded surface due to temperature change at the heat treatment can be made small, so that delamination of the bonded surface resulting from the thermal stress can be prevented, and the optical device bonded with sufficient bonding strength can be manufactured. Moreover, it becomes possible to bond the optical elements, even if the flatness and the surface roughness of the optical elements are not fully controlled. Moreover, since they are bonded without using organic adhesive, there is no generation of outgas. Furthermore, since bonding state of the optical device bonded as described above is excellent, the forward direction insertion loss is also low, and it has excellent optical properties. Therefore, an optical device which is small size, reliable and highly efficient can be manufactured at a low cost.
In that case, it is preferable to stick the optical elements each other with heating the optical elements so that temperature of the optical elements may be 40xc2x0 C. or more and 100xc2x0 C. or less.
If the optical elements are bonded each other with heating so that the temperature of the optical elements may be 40xc2x0 C. or more as described above, thermal stress generated due to the temperature change at the heat treatment can be made smaller, and delamination of the bonded surface due to thermal stress can be prevented effectively. Moreover, if the temperature of the optical elements is 100xc2x0 C. or less, the optical elements can be stuck easily, without generation of problems on work.
In that case, it is preferable to keep the temperature of the optical elements at 40xc2x0 C. or more after sticking the optical elements until they are subjected to the heat treatment.
If the temperature of the optical elements is kept at 40xc2x0 C. or more after the optical elements are stuck until they are subjected to the heat treatment as described above, thermal stress generated due to lowering of the temperature of the optical element from sticking the optical elements each other to subjecting them to the heat treatment can be reduced, and delamination of the bonded surface of the bonded body stuck will weak bonding strength can be prevented. Moreover, by subjecting the bonded body to the heat treatment thereafter, bonding strength of the bonded surface can be increased further, and the optical device bonded with sufficient bonding strength and a excellent bonding state can be manufactured.
Moreover, in that case, it is preferable to stick the bonded surfaces directly or through solution when the above-mentioned optical elements are stuck at the bonded surface.
If the optical elements are bonded at the bonded surface each other directly or through solution as described above, the chemical species which constitute each of the optical elements can interact effectively, and the optical elements are bonded each other with high bonding strength.
Moreover, in case that the above-mentioned optical elements are stuck each other through solution, as the solution it is preferable to use liquid containing polar molecules as a main component independently or by mixture.
If the liquid containing polar molecules as a main component is used independently or by mixture when the optical elements are stuck through the solution as described above, bonding strength to the optical elements can be increased further and delamination which generated in the bonded surface can be effectively prevented.
Moreover, in the heat treatment process after sticking the above-mentioned optical elements each other on the bonded surface, the heat treatment temperature is preferably 100xc2x0 C. or more and 400xc2x0 C. or less.
If the heat treatment temperature of the heat treatment process is 100xc2x0 C. or more and 400xc2x0 C. or less as described above, the bonding strength of the optical elements can be increased effectively, and the optical device bonded with sufficient bonding strength and having an excellent bonding state can be manufactured.
In case that the above-mentioned heat treatment is performed, a rate of increasing temperature is preferably 20xc2x0 C./hr or less.
If the rate of increasing temperature is 20xc2x0 C./hr or less as described above, large thermal stress is not rapidly generated on the bonded surface when performing the heat treatment. Therefore, delamination of the bonded surface can be reduced and variation in bonding strength at the bonded surface call be further made small. Therefore, there can be obtained the optical device in which optical elements are bonded each other in a good state,
Furthermore, it is preferable that the above-mentioned heat treatment is performed in the low temperature atmosphere or in the atmosphere containing hydrogen.
If the heat treatment atmosphere is the low pressure atmosphere or the atmosphere containing hydrogen as described above, bonding strength of the bonded surface can be further increased.
In that case, the optical elements to be bonded can be at least a magnetic garnet crystal and a polarizer.
If the optical elements are tit least a magnetic garnet crystal and a polarizer as described above, the resultant optical device can be the optical device which functions as an optical isolator. The optical isolator is one of those having the highest utility value among optical devices, and it is an indispensable device in optical communication. Therefore, according to the present invention, the optical isolator which is small size and has sufficient bonding strength can be provided.
Furthermore, in that case, it is preferable that a metal oxide film is previously formed on the bonded surface of at least one of the optical elements before sticking the optical elements each other, and then the optical elements are stick each other.
If the metal oxide film is formed on the bonded surface of at least one of the optical elements to be bonded, for example, in the case that a magnetic garnet crystal and a polarizer are bonded, on the bonded surface of the magnetic garnet crystal which is to be bonded to the polarizer, and then they are stuck together, the bonding strength of a bonded surface can be further increased. Moreover, since the formed metal oxide film functions as an antireflection film in the optical device, a reliable and highly efficient optical device can be manufactured.
In that case, it is preferable that the metal oxide film formed on the bonded surface of the optical element consists of one kind or two or more kinds of metal oxide films selected from Al2O3, TiO2 and SiO2, and the metal oxide film is laminated in a single-layer or a multilayer.
If the metal oxide film to be formed consists of one kind or two or more kinds of metal oxide films selected from Al2O3, TiO2 and SiO2, and it is laminated in a single-layer or a multilayer as described above, the function as an antireflection film of the metal oxide film can be enhanced further, and the bonding strength of the optical elements can also be increased remarkably.
The optical device manufactured by the production method according to the second embodiment or the present invention is formed by bonding the optical elements each other with sufficient bonding strength without using organic adhesive. Thus, outgas is not generated, and degradation of a bonded surface is not caused. Furthermore, since the bonding state of a bonded surface is also excellent, the forward direction insertion loss of the optical device can be reduced. Therefore, there can be obtained an optical device which is small size, and has high reliability and high performance.
Moreover, the optical device produced by the production method according to the second embodiment of the present invention can be, for example, an optical isolator.
As described above, the optical isolator is one of the optical devices which has the most valuable utility. Accordingly, when the optical device of the present invention is an optical isolator, there can be provided an optical device which can meet miniaturization of an optical isolator and an organic adhesive free optical device strongly requested in recent years. Furthermore, since it has sufficient bonding strength and the bonding state thereof is also excellent, there can be provided a highly reliable and highly efficient optical isolator.
As explained above, according to the present invention, optical elements can be easily bonded each other with strong bonding strength without using adhesive. Furthermore, there can be provided a small size and highly reliable optical device having excellent optical properties at a low cost in which there is neither generation of outgas nor degradation of the bonded surface.